1. Field of the Invention
The present invention relates to an electrode for electrochemical measurement used in sensors for detecting specific substances, and a method for manufacturing the same.
2. Description of the Related Art
Conventionally, attempts have been made to use carbon nanotubes in electronic devices because of their semiconductive characteristics.
The advantage of using carbon nanotubes as electronic devices is their extremely high conductivity. Their small diameters of 1 to 20 nm or so make them suitable for use as devices and electrodes in micro circuits.
On the other hand, from the viewpoint of application to the medical field, biosensors for detecting trace amounts of biological substances are being heavily studied and developed. Detecting methods in actual use can be classified into three methods: (1) an electrochemical method, (2) an enzymatic method, and (3) an staining method.
Of these, the electrochemical method is the most general detecting method. Substances have oxidation-reduction potentials unique thereto, and by applying a specific potential, electrons can be pulled out of (oxidation) or injected into (reduction) a substance. Therefore, the electron exchanges involved in the oxidation-reduction reactions can be measured by cyclic voltammetry or the like so as to find the amount of the target substance from the current value at a certain potential.
In the electrochemical method, micro electrodes are used to improve detecting sensitivity. The micro electrodes refer to electrodes having sizes of a micrometer level or smaller, while the electrodes generally used for electrochemical measurement have sizes of several millimeters to several centimeters (see, for example, S. Pons and M. Fleishmann, Analytical Chemistry, 1987, volume 59, page 1391A). The use of such micro electrodes has the following features (1) to (4).    (1) The provision of charging current which is noise can be reduced.    (2) Potential can be scanned at a high speed.    (3) The effects of substance dispersion can be reduced.    (4) High sensitivity measurement can be accomplished.
Due to these features, the use of micro electrodes is becoming the mainstream in detecting trace amounts of samples by the electrochemical method.
However, metal electrodes such as platinum and gold and carbon electrodes which are generally used in the electrochemical method have a narrow potential window, which causes a problem in that applying a potential higher than a certain level electrolyzes not only the target substance but also the solvent itself, hindering detection of the target substance. This requires using different electrodes depending on the potential to be used, making it impossible to measure cyclic voltammetries at the same time with a wide potential. In recent years, as an electrode with a wide potential window capable of solving this problem, an electrode containing boron-doped diamond has been developed. However, it has not been put into practical use as a general-purpose product because of its high cost.
On the other hand, the enzymatic method is a method for detecting a target substance electrochemically by using an electrode having an enzyme fixed on its surface. Enzymes have the feature of being capable of selectively detecting a target substance at comparatively high sensitivity from a mixture because they react uniquely with the target substance. So far, glucose sensors (diabetes testing), uric acid sensors (gout testing), and urea sensors (kidney function testing) are already in actual use in the medical field. However, there is a problem in that enzymes are difficult to handle because they are unstable and must be stored under special conditions to maintain their activity.
The staining method is a method for detecting a biological substance by measuring the ultraviolet-visible absorption spectrum using a reagent which is colored in a reaction with the target substance, and by finding its absorbance. However, the detecting sensitivity in the absorbance measurement is proportional to the light path length, so that to improve the sensitivity requires a large amount of sample solution. Thus, there is a problem in that the method cannot be applied for the detection of trace amounts of samples.
Specific examples of sensors utilizing the electrochemical detecting method include: sensors for detecting a specific substance by using ion sensitive field-effect transistors (see, for example, Japanese Patent Application Laid-open (JP-A) No. 03-272449); sensors for detecting hydrogen peroxides by using an electrode with ferrocene fixed on its surface (see, for example, C. Padeste et. al, “Ferrocene-avidin conjugates for bioelelectrochemical applications,” Biosensors & Bioelectronics, 2000, volume 15, pp. 431-8); and sensors for detecting a specific substance by using a carbon nanotube as an electrode and making use of changes in electrical properties of the carbon nanotube due to external stimulation (See, for example, JP-A No. 2003-227808). However, these sensors are unsatisfactory in terms of the aforementioned viewpoint, and improvements are still expected.